Battery controllers are designed to compare the voltage of a battery with a threshold voltage to thereby prevent overcharging or overdischarging of the battery. One type of these battery controllers is installed with a fault diagnosis function of monitoring a spontaneous change in the threshold voltage due to, for example, the deterioration of electric elements with time; these electric elements configure a voltage source of the threshold voltage. One example of this type of these battery controllers is disclosed in Japanese Patent Application Publication No. 2003-92840.
The fault diagnosis function of the battery controller disclosed in the Patent Publication forcibly switches the threshold voltage relative to a specific value of the voltage of a battery by only a preset level to the higher side or the lower side, and compares the specific value of the battery voltage with the switched threshold voltage.
When the magnitude relation between the specific value of the battery voltage and the forcibly switched threshold voltage is not inversely related to that between the specific value of the battery voltage and the threshold voltage, the fault diagnosis function determines that there is a great spontaneous change in the threshold voltage. This detects the occurrence of the characteristic deviation of the threshold voltage from its original value.